Mechanical assist ventilation is delivered to a patient in order to maintain adequate alveolar ventilation by partially or totally substituting the function of the presumably weak or failing respiratory muscle(s). Initially, it was found preferable to deliver mechanical assist ventilation to a patient by applying a negative pressure around the patient's ribcage and/or abdomen; the so-called negative pressure assist ventilation. In fact, negative pressure assist ventilation creates a partial vacuum around the patient's ribcage and/or abdomen to induce inspiration. This mode of mechanical assist ventilation proved useful, but restricted physical access to the patient during ventilation.
Nowadays, mechanical assist ventilation is delivered, with a few exceptions, by applying a positive pressure to the patient's airways; the so-called positive pressure assist ventilation. In order to completely unload the patient's inspiratory muscles with positive pressure assist ventilation, it is necessary to apply a positive pressure that is capable not only to inflate the lungs, but also to displace the chest wall and the abdomen. In practice, this requires application of a positive pressure exceeding the transpulmonary pressure; the transpulmonary pressure is the pressure required to inflate the patient's lungs alone. A high inspiratory positive assist pressure increases the risk for barotrauma and pneumothorax, and can have a negative effect on hemodynamics.
Also, excessive inspiratory loads imposed by the patient's abdomen and/or ribcage may occur with obesity, reduced compliance due to abdominal distension, dynamic hyperinflation, and/or deformities of the chest wall. Obviously, higher inspiratory positive assist pressure supplied to the patient's airways is required to overcome these additional inspiratory loads.
Finally, the current commercially available modes of both positive and negative pressure assist ventilation presents the following limitation: the pneumatic system, designed to respond to the patient's effort, induces a time lag that can influence the patient's breathing pattern, leading to deterioration of the patient-ventilator synchrony. This situation is often referred to as the patient “fighting the ventilator”. In order to avoid the patient “fighting the ventilator”, increased patient sedation and even respiratory muscle paralysis is often required.